(a) Technical Field
The present invention relates to a plasma nitriding surface treatment method for a gray cast iron part. More particularly, it relates to a plasma nitriding surface treatment method for a gray cast iron part, which can improve the exterior of the part by preventing the occurrence of rust.
(b) Background Art
Generally, brake discs serve to stop vehicles by changing kinetic energy of a moving vehicle into thermal energy by friction between the discs and frictional materials. As such, brake discs require very efficient heat radiation capacity.
In order to meet these requirements, brake discs are primarily being manufactured using gray cast iron, which is a cheap material having sufficient heat radiation characteristics.
A brake disc formed of gray cast iron, which is a frictional material, may undergo nitriding treatment to increase abrasion resistance.
FIG. 1 is a view illustrating a method of oxy-nitriding a brake disc according to a related art. In the oxy-nitriding method of a brake disc 10, the brake disc 10 is put into a furnace, and undergoes an oxy-nitriding treatment for 4 to 5 hours at a temperature of 500° C. to 700° C. under an atmosphere where oxygen (O2) is added to ammonia gas (NH3) to form an oxy-nitride layer on the surface of a gray cast iron.
For example, a process of forming an oxy-nitride layer as shown in FIG. 1 includes performing a nitriding process for 250 minutes at a temperature of 610° C. under an atmosphere of ammonia (NH3), nitrogen (N2), and carbon dioxide (CO2), performing an oxidizing process for 20 minutes at a temperature of 555° C. using water vapor, and performing natural air cooling.
Table 1 shows the thickness variation of the brake disc 10 oxy-nitrided by the above method before and after the oxy-nitriding.
TABLE 1AfterItemSampleAfter grindingoxy-nitridingVariationDisc thickness128.02028.0400.020measurement228.02028.0500.030(Thickness:328.02028.0400.02028 mm)428.02028.0500.020528.02028.0500.030
As shown in Table 1, the thickness of the brake disk 10 increase by about 20 μm to about 30 μm after the oxy-nitriding due to a porous layer.
Upon oxy-nitriding, a nitride layer of an epsilon (ε) phase is formed on the surface by a reaction of gray cast iron and atmospheric oxygen. This nitride layer of the epsilon phase increases brittleness. Furthermore, due to the porous layer, corrosion on the frictional surface becomes severe, and surface peeling occurs upon braking.
The above information disclosed in this Background section is only for enhancement of understanding of the background of the invention and therefore it may contain information that does not form the prior art that is already known in this country to a person of ordinary skill in the art.